The present invention relates to antenna systems, and more particularly to a reflector antenna adapted to be disposed on an exterior surface of a moving platform such as an aircraft, and further which includes certain signal processing components being located closely adjacent to an antenna aperture on an exterior surface of the mobile platform and certain signal processing components being located within the interior of the mobile platform.
Antenna systems are used in a variety of applications. One application which is growing in importance is in connection with satellite linked communication systems for providing Internet connectivity with mobile platforms such as aircraft. In such applications, the antenna system disposed on the aircraft must present a package which is low in height and width when mounted on an exterior surface of the fuselage of the aircraft so that the antenna system does not adversely affect the aerodynamics of the aircraft. Nevertheless, such antennas must still provide a high gain/temperature (G/T) and include an antenna aperture which is capable of being rotated along an azimuthal axis as well as an elevation axis such that the antenna can be pointed in a desired direction.
Still another consideration with such antennas is the location of certain signal processing components. It would be desirable to locate certain signal processing components within the interior of the mobile platform. This would make such components easily accessible in the event repair or maintenance is required on the antenna system. Conversely, it would be desirable to locate other components, such as low noise amplifiers, close to the antenna aperture. This would help to ensure that the antenna achieves a high G/T.
With reflector antennas such as a cassegrain system, an additional problem is posed with the length of the feedhorn employed. The feedhorn may need to have a particular length which is required to efficiently illuminate the sub-reflector and to minimize the spillover energy pass the sub-reflector which provides high sidelobes in the transmit and receive antenna patterns. However, the feedhorn must still be short enough such that it does not create an antenna which has an unacceptably high profile, and thus an unacceptable aerodynamic drag and if disposed on fast moving mobile platforms such as jet aircraft.
It is therefore a principal object of the present invention to provide an antenna system which is particularly well adapted to be mounted on an exterior surface of a mobile platform, such as an aircraft, and which presents a low profile which is aerodynamically efficient. It is a further object of the present invention to provide such an antenna system which includes certain components mounted exteriorly of the mobile platform and certain other components which are mounted inside the mobile platform. In this manner, those components which need to be located physically close to the antenna aperture to maximize antenna performance can be so located, while other components which do not need to be located close to the antenna aperture can be disposed within the interior of the mobile platform for easy servicing and/or maintenance.
The above and other objects are provided by a transmit/receive (TX/RX) reflector antenna system in accordance with a preferred embodiment of the present invention. The TX/RX reflector antenna system includes an antenna aperture comprised of a main reflector, a sub-reflector and a feedhorn. The feedhorn is disposed within an aperture at an axial center of the main reflector such that a portion of the feedhorn extends forwardly of the main reflector while a portion extends rearwardly of the main reflector. In this manner, a longer feedhorn can be employed without producing an antenna that has an unacceptably large, cross-sectional profile which would therefore be aerodynamically inefficient on a fast moving mobile platform such as a jet aircraft.
In one preferred embodiment a first antenna signal processing subsystem is disposed closely adjacent to the antenna aperture exteriorly of the mobile platform under a radome, while a second antenna signal processing subsystem is disposed within the interior of the mobile platform. The two subsystems are coupled by a rotary joint, which in one preferred form comprises a two channel coaxial rotary joint. The first antenna signal processing subsystem includes two pairs of diplexers. The first pair is used to process vertically polarized RF energy while the second pair is used to process horizontally polarized RF energy. A suitable transducer in communication with the feedhorn splits circularly polarized (RHCP and LHCP) RF signals received by the antenna aperture into vertical and horizontal components for signal processing. In addition, the transducer, during a transmit function, accepts vertical and horizontal components of variable phase angle which are fed into the feedhorn to produce a linear polarization with variable angle.
The second antenna signal processing subsystem also includes a third pair of diplexers. One of this third pair of diplexers is used in a transmit subsystem and the other of the third pair is used in a receive subsystem. The transmit subsystem further includes at least one high power amplifier along with at least one phase shifter for amplifying and phase shifting a transmit signal being sent to the antenna aperture. The receive subsystem includes at least one bandpass filter for filtering signals received by the antenna aperture. Each of the transmit and receive subsystems further includes a hybrid circuit for interfacing with one of a transmit input or a receive output of the second antenna signal processing subsystem.
The first antenna signal processing subsystem further includes at least one, and preferably a pair, of low noise amplifiers. The low noise amplifiers are disposed closely adjacent to the main reflector to thus enable the antenna system to achieve a high gain/temperature (G/T). The high power amplifiers of the second antenna signal processing subsystem are disposed within the mobile platform and are thus available for convenient access in the event of needed maintenance or service. Locating the components of the second antenna signal processing subsystem within the mobile platform further helps to limit the physical size of the antenna structure which must be disposed on the exterior of the mobile platform, and thus helps to ensure that the aerodynamics of the mobile platform are not adversely affected by the presence of such components.